For much of Wednesday's episode, which coincided with the Supergirl, The Flash and Legends of Tomorrow crossover event, Oliver (Stephen Amell) spent a large part of it stuck in an alien pod -- "the Dominator matrix," as executive producer Marc Guggenheim called it -- and hallucinating what his life would have been like had he not gotten on the Queen's Gambit all those years ago.

Let's just say, Oliver's life would have been very different -- no crime-fighting, for one thing -- and far less tragic.

In the faux reality, Oliver and Laurel (Katie Cassidy) were engaged, Ray (Brandon Routh) and Felicity (Emily Bett Rickards) were about to walk down the aisle, Moira (Susanna Thompson) and Robert (Jamey Sheridan) were alive and happy together, Diggle (David Ramsey) was the Green Arrow, and Sara (Caity Lotz) came into town for her sister's wedding.

"We make a reference in the end of the Legends episode [of] the crossover that this was sort of Oliver's version of 'Flashpoint' and it's Oliver's version of 'The Girl Who Has Everything' [episode] from last year's Supergirl," Guggenheim told reporters at a recent screening. "There's a reason why these stories are iconic or familiar tropes, even in comic books."

"When you show the protagonist the path not taken and you put them in the situation where they can choose to stay on that path or go back to their life with all of its ugly aspects and challenges, and they choose the selfless choice of returning to that ugly past," he said, "it makes your character stronger because it forced Oliver to double down on his mission and commit to this life with all of its losses and failures and challenges."

Oliver and Laurel's engagement served as the structural backbone of the episode, giving a reason for familiar faces to come together for a momentous -- albeit fake -- occasion. The Arrow producers explained why they felt it important to make Laurel, who was killed off in season four, a central figure in such a pivotal episode.

"Even though the show has evolved, Laurel is at the heart of it. She was Oliver's great love, she's Sara's sister, she's Lance's daughter. Watching those early episodes, so much of it revolved around her relationship with all these characters," executive producer Andrew Kreisberg said. "Even if we do get to 200 episodes, Laurel will always be at the heart of the series and be an important character."

"On top of that, Katie Cassidy will always be so important to us," he continued. "We were so happy that she agreed to come back because she is part of the family, both behind the camera and in front of it."

While it was bittersweet seeing Thompson and Sheridan back in the Arrow fray, not all the actors the producers wanted to bring back for the landmark moment were available -- namely Colton Haynes (Roy) and Colin Donnell (Tommy).

"We're so immersed in it, sometimes it's easy to forget just how far the show has come and how different it used to be," Kreisberg said. "Watching the early episodes and watching the 100th and seeing Oliver have scenes with his mom, and remembering how important [Moira] was to the mythology and these characters and to everything that happened, that was the thing that made it feel like a 100th episode."

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That's not to say Haynes and Donnell's presence wasn't felt in the episode. There was a cheeky reference to Donnell's current role on NBC's Chicago Med during the engagement party scene, where Tommy's absence was explained as him working "triple shifts at the hospital."

Holograms of Roy and Tommy -- as well as Moira, Robert, Felicity and Laurel -- appeared before Oliver, right before he left his faux reality and returned to his real life. "That beat was not in the original break of the episode," Guggenheim told ET, revealing that Haynes and Donnell's holograms were taken from old episodes and modified to fit the scene.

Guggenheim shared that there were scripted moments with Haynes and Donnell that were in the script "that we obviously couldn't do" due to their "availabilities." One of those moments had Roy playing a significant part in the faux reality. "Roy was going to be Thea's boyfriend -- that hadn't changed -- and they met when he stole her purse, and that also hadn't changed," he said. "I thought that would have been fun and nice to see."

As for what the events of the 100th episode mean for the next week's fall finale, Guggenheim reaffirmed that Oliver "has a new sense of purpose." "He goes into [the episode] with a reaffirmation of his bond with Thea because they basically chose each other," he hinted. "That carries through the midseason finale and sets up things beyond it."

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